The present invention relates generally to the shape of the front convex surface of an aspherical spectacle lens and, in particular, to the shape of an aspherical spectacle lens having a positive power.
Generally, the front convex surfaces of spectacle lenses for correcting the refractive error in eyes have been made spherical for ease in machining the lenses. Such a lens is called a spherical lens. Generally, the refractive power of a lens is represented in diopters (D). The refractive power of a surface of a lens is defined as EQU (n-1).times..rho.
where .rho. is the curvature (m.sup.-1) of the surface and n is the refractive index of the material of the lens. The refractive power of the front refractive surface of a lens is better known as the base curve. The curvature corresponding to the base curve is hereinafter referred to as the curvature of the base curve.
Since the power of a lens is mainly determined by the refractive powers of the front surface and the rear surface of the lens, the base curve can assume various values, depending on the combination of the refractive powers for a given value of lens power. In practice, however, the base curve is restricted to a certain range by the power of the lens because of the optical performance, especially to reduce the astigmatism produced on the eye when an object is viewed through the side portion of the lens which is at a distance from the Optical axis of the lens.
One example of this is shown in FIG. 2 where the base curve is plotted on the vertical axis and the power of a lens having a refractive index of 1.50 is plotted on the horizontal axis. This graph shows the astigmatism produced when the spectacle lens is actually used and an object located 30.degree. to the optical axis is viewed. The solid lines indicate the astigmatism produced when a distant object is viewed The numerical values adjacent the lines indicate the amounts of astigmatism Lines indicating astigmatism of 0.3 D are shown on opposite sides of a line indicating the absence of astigmatism, i.e., O D. The broken lines indicate the astigmatism produced when an object located at a short distance of 30 cm is viewed.
As can be seen from this graph, the optimum base curve giving zero astigmatism differs between when a remote object is viewed and when a close object is viewed. Accordingly, the base curve a in the hatched region is generally selected so as to be able to view remote objects and close objects alike.
Conventional lenses which have positive powers and are principally used for far-sighted persons and presbyopic persons have several disadvantages. In particular, lenses having larger powers have larger thicknesses at their centers. As the degree of the farsightedness or presbyopia increases, a lens having a base curve having larger curvatures must be employed, and the convex surface protrudes more This is not desirable from an aesthetic point of view.
FIG. 3 is a cross section of one example of such a lens. The illustrated lens has a power of +3 D and a diameter of 72 mm The lens is a generally used plastic lens having a refractive index of 1.50. The base curve is 7.5 D, and the thickness at each edge is 1.0 mm. In this example, the thickness at the center of the lens is 5.3 mm. The amount l by which the convex surface of the lens protrudes from the edges of the lens is 10.6 mm. If spectacles are fabricated from lenses of this construction, then the lenses are considerably thick and unsightly. One conceivable method of solving this problem is to reduce the base curve.
FIG. 4 shows a lens which is similar to the lens shown in FIG. 3 except that the base curve is 4.0 D. The thickness at the center of this lens is 4.9 mm, which is less than the thickness of the lens shown in FIG. 3 by 0.4 mm. Also, the amount of protrusion is 5.3 mm, which is half of the amount of the lens shown in FIG. 3. However, the base curve is determined from the optical performance as discussed above.
As shown by the graphs in FIGS. 5 and 6, the base curve of 4.0 D severely deteriorates the optical performance. FIGS. 5 and 6 show the astigmatism produced in the field of view when a lens having a base curve of 7.5 D and a lens having a base curve of 4.0 D are respectively used. The vertical axis indicates the angle of the field of view in degrees, while the horizontal axis represents the astigmatism in diopters, measured based on the refractive power of the sagittal direction. The astigmatism occurring in fields of view when an infinitely remote object (.infin.), an object at a distance of 1 m, and an object at a distance of 0.3 are viewed, are shown.
To solve the above-described disadvantages, some lenses having aspherical front refractive surfaces have been proposed, as disclosed for example in Japanese Patent Laid-Open No. 136,644/1977, Japanese Patent Publication No. 15,248/1985 corresponding to U.S. Pat. No. 4,181,409, and Japanese Patent Laid-Open No. 24,112/1983 corresponding to U.S. Pat. No. 4,504,128. In the lens disclosed in Japanese Patent Laid-Open No. 136,644/1977, a meridian is formed by a quadratic curve such as an ellipse, parabola, or hyperbola. The front refractive surface is formed by an aspherical surface that is created by rotating the meridian. A plurality of lenses of this type have been suggested.
Japanese Patent Publication No. 15,248/1985 and Japanese Patent Laid-Open No. 24,112/1983 disclose lenses having large positive powers, the lenses being used for aphakial eyes. The lens disclosed in Japanese Patent Publication No. 15,248/1985 adopts an aspherical surface of revolution based on an aspherical surface of revolution of a tenth-order function of the radius r. The lens disclosed in Japanese Patent Laid-Open No. 24,112/1983 adopts an aspherical plane of revolution based on a quadratic curve. A correcting term is added to it. What is common to these conventional aspherical lenses is that the curvature of the meridian decreases substantially monotonously land acceleratingly from the axis of rotation (generally the geometrical center of the lens) toward the edges. As a result, the power of the lens is much lower in the peripheral portions than in the center. This narrows the effective field region suitable for the condition of the user's eye. Especially, a lens for an aphakial eye has a strong aspherical surface to make the lens thin and so the diameter of the effective field region is from 30 to 40 at best on the lens.
Accordingly, it is desired to provide an aspherical spectacle lens that overcomes the disadvantages of the prior art.